1. Field of the Invention
The present invention generally relates to a bicycle frame. More particularly, the present invention relates to a bottom bracket support of a bicycle frame.
2. Description of the Related Art
Bicycles with frames fabricated from oversized aluminum tubing have become increasingly popular. Unlike steel, aluminum cannot be brazed, so that joints between the tubes of most aluminum bicycle frames have to be welded. A critical joint in the manufacture of modern bicycle frames is the joint between the head tube, the top tube, and the down tube. The fork acts as a long lever arm and can exert significant amounts of stress on the head tube. The arrival of suspension bikes in the market place with stiff long-travel suspension forks has made the design of this junction even more critical.
Top tubes and down tubes have been getting bigger to achieve greater strength and rigidity. This has created problems in trying to accommodate the larger top and down tubes. Commonly, the top and down tubes are down sized at the head tube end to mate with a standard sized head tube. However, this reduces the effectiveness of the oversized tubing use for the top tube and down tube. An alternative approach has been to increase the diameter of the head tubes and the associated steer tube bore. While the larger diameter head tube avoids the need to crimp the top and down tube, the approach requires nonstandard bearings and can require a nonstandard steer tube. Significantly, this approach can add undesired weight, which is directly contrary to the desires of the market. Typically, manufacturers have accommodated these larger top and down tubes by crimping the tubes at their juncture with the head tube. This obviously creates strength and repeatability issues at the juncture.
The bicycle frames with oversized down tubes and top tubes are traditionally constructed such that one of the top tube and down tube is mitered to the head tube only, and the other of the top tube and down tube is mitered to both the head tube and the down tube. This method of manufacture makes it more difficult to use complex cross sectional frame tubing. Further more, having to crimp down the end of the top and down tubes and then having to perform these complex cuts makes the situation even more difficult. This type of cutting process needs either expensive equipment if the cutting process is automated or skillful operators if the cutting process is done manually.
Another important joint in the bicycle frame is between the down tube and the bottom bracket shell. In addition to supporting a pedal crank assembly, the bottom bracket shell is often used to connect a down tube of the main frame to a seat tube of the main frame. If the bottom bracket shell is a cylindrical tube, as is common, the surface area available to receive and support the seat tube and down tube is limited. As a result, the diameters of the seat tube and down tube may be limited by the size of the bottom bracket shell, which generally is sized to receive an industry-standard bottom bracket assembly. As noted above, larger down tubes are desirable to improve strength and rigidity of the bicycle frame.